The objective of this research is isolation and biological characterization of soluble factor(s) which apparently obviate(s) the requirement for thymus derived (T) lymphocytes in the in vitro induction of certain "thymic dependent" immune responses. Such factor(s) have been found in certain batches of fetal calf sera (FCS) and in media conditioned by the culture of fetal mouse thymuses. The sera restore the heterologous (sheep, horse, donkey) erythrocyte responses of neonatally thymectomized (NTX), adult thymectomized, lethally x-irradiated and bone marrow protected (ATXBM) and congenitally athymic "nude" mice. They also restore cytotoxic lymphocyte activity of NTX and ATXBM mouse spleen cells in a completely in vitro system for the generation of allogeneic "killer" cells. These sera do not restore the responsiveness of spleen cells from these animals to the lectin phytohemagglutinin (PHA). The isolation objective will be accomplished using a bioassay system designed to measure the restoration of the SRC response of NTX spleen cells cultured in sera which do not contain the factor(s). Separation of the active material will be achieved using column chromatography techniques. Once the separation has been accomplished, investigations will be undertaken to determine whether the factor functions by amplification of the T cell residium or by overcoming the T cell requirement. The isolated material will also be assayed in vivo in NTX animals to determine its effect on skin graft survival, graft-versus-host activity, wasting syndrome, peripheral blood lymphocyte levels and the cellularity of the thymic dependent areas of spleen and nodes.